A new form of the contraceptive pill could cut the number of periods a woman has in a year by two-thirds.

It could also help to reduce the symptoms of pre-menstrual syndrome.

US researchers have almost completed trials of the pill, which could cut a woman's monthly periods to just four a year.

If things go according to plan, the pill, named Seasonale, could be on the market by next year.

The pill, developed by a team from Eastern Virginia Medical School, extends the monthly menstrual cycle to three months.

Combined pill

Seasonale is not a new birth control pill, but a combination of two commonly used hormones already available in various dosages in oral contraceptives.

Women would take it for 84 consecutive days and then
skipping a week.

A spokeswoman for the Medical School said the trials, which began nearly two years ago and which have been carried out at the school and at 49 other sites across the US, are practically finished and the data now has to be analysed.

The Food and Drug Administration must approve Seasonale before it goes on the market.

Researcher Dr Freedolph D Anderson said side effects should be the same as for any birth control pill.

He said: "Low and ultra-low doses are used, so the side effects should be minimal.

"The main advantages are fewer periods per year - four versus 13 - less blood loss, fewer pads, and less risk of the side effects of menstruation such as
anaemia.

"Moreover, the hormonal fluctuations typical of a real menstrual cycle are smoothed out and this may help to relieve pre-menstrual tension syndrome."

Pregnancy

Medical experts say the new pill will not harm a woman's chance of becoming pregnant once she stops taking it.

Contraceptive pills work by artificially raising the level of female hormones in the blood.

This overrides the normal hormonal signal from the ovaries and prevents the shredding of the lining of the uterus which is the bleeding seen during menstruation.

A spokeswoman for the Family Planning Association said doctors already advised some women for medical reasons to run two or three packs of the combined pill together to reduce their number of periods.

She said: "There is no evidence at the moment that this does any harm to a woman's chances of becoming pregnant.

"We welcome Seasonale as another choice for women, and we look forward to when it becomes available in the UK."